1. Field of the Invention
The present invention relates to optical projection systems, and in particular to a high variable numerical aperture, large-field unit-magnification projection optical system.
2. Description of the Prior Art
Photolithography is presently employed in sub-micron resolution integrated circuit (IC) manufacturing, and also to an increasing degree in advanced wafer-level IC packaging as well as in semiconductor, microelectromechanical systems (MEMS), nanotechnology (i.e., forming nanoscale structures and devices), and other applications. These applications require multiple imaging capabilities from relatively low (i.e., a few microns) resolution with large depth of focus, to sub-micron resolution and a high throughput.
The present invention, as described in the Detailed Description of the Invention section below, is related to, and an improvement on, the optical system described in U.S. Pat. No. 4,391,494 (hereinafter, “the '494 patent”) issued on Jul. 5, 1983 to Ronald S. Hershel and assigned to General Signal Corporation, which patent is hereby incorporated as a reference.
FIG. 1 is a cross-sectional diagram of an example prior art optical system 8 according to the '494 patent. The optical system described in the '494 patent and illustrated in FIG. 1 is a unit-magnification, catadioptric, achromatic in the g-h band and is an anastigmatic optical projection system that uses both reflective and refractive elements in a complementary fashion to achieve large field sizes and high numerical apertures (NAs). The system is basically symmetrical relative to an aperture stop located at the mirror, thus eliminating odd order aberrations such as coma, distortion and lateral color. All of the spherical surfaces are nearly concentric, with the centers of curvature located close to where the focal plane would be located were the system not folded. Thus, the resultant system is essentially independent of the index of refraction of the air in the lens, making pressure compensation unnecessary.
Optical system 8 includes a concave spherical mirror 10, an aperture stop AS1 located at the mirror, and a composite, achromatic piano-convex doublet lens-prism assembly 12. Mirror 10 and assembly 12 are disposed symmetrically about an optical axis 14. Optical system 8 is essentially symmetrical relative to an aperture stop AS1 located at mirror 10 so that the system is initially corrected for coma, distortion, and lateral color. All of the spherical surfaces in optical system 8 are nearly concentric.
In optical system 8, doublet-prism assembly 12 includes a meniscus lens 13A, a piano-convex lens 13B and symmetric fold prisms 15A and 15B. In conjunction with mirror 10, assembly 12 corrects the remaining optical aberrations, which include axial color, astigmatism, petzval, and spherical aberration. Symmetric fold prisms 15A and 15B are used to attain sufficient working space for movement of a reticle 16 and a wafer 18.
Optical system 8 also includes an object plane OP1 and an image plane IP1, which are separated via folding prisms 15A and 15B. The cost of the gain in working space is the reduction of available field size to about 25% to 35% of the total potential field. In the past, this reduction in field size has not been critical since it has been possible to obtain both acceptable field size and the degree of resolution required for the state-of-the-art circuits.
However, most present-day (and anticipated future) high-technology micro-fabrication processes (e.g., for wafer-level IC packaging, semiconductor fabrication, forming MEMS and nano-structures, etc.) include performing a large number of exposure steps using 200-mm and 300-mm wafers. Further, the exposures must be performed in a manner that provides a large throughput so that the fabrication process is economically feasible.
Unfortunately, the optical system of the '494 patent is not capable of providing high-quality imaging at large field sizes (e.g., from three to six 34×26 mm step-and scan fields) with minimum resolution ranging from 0.75 micron to 1.4 microns. Such performance is necessary for, among other things, so-called “mix-and-match” applications, wherein different masks requiring different resolutions (which, in turn typically requires different photolithographic systems) are used in the microdevice fabrication process.
It would be advantageous to have a projection optical system capable of providing large-field, lower-resolution imaging, and moderate size field, high-resolution imaging. In order to reduce the large number of exposure steps required by the '494 patent associated with the 200-mm and 300-mm wafers, it is necessary to develop a large-field projection lens capable of imaging multiple step-and-scan fields (34 mm×26 mm) or multiple step-and-repeat fields (22 mm×22 mm). This would provide increased system throughput in these applications. Accordingly, there is an increasing demand for a high NA large-field, variable numerical aperture, high throughput projection photolithography system capable of low and high resolution imaging suitable for bump technology as well as for Mix-and-Match applications. The present invention solves those problems and provides such a projection optical system.